The present invention relates to the field of heating elements of the sheathed type in which a resistance wire encased in an insulator is housed in a metal tube. The insulator may be made of a material such as magnesia. The present invention relates in particular to elements of this type having particular electrical characteristics.
It is known, in appliances of the water heater type, to utilize resistance heating elements whose resistance presents a significant thermal coefficient, i.e. experiences a significant increase in resistance when the temperature increases. This characteristic is known as a positive temperature coefficient (PTC).
This characteristic is expressed by the formula:ρ=ρo[1+α(T−25)]where ρo is the resistivity of the wire at 25° C., ρ is the resistivity of the wire at the temperature T expressed in ° C., and α is the temperature coefficient.
This property results in a reduction in the power converted to heat, or dissipated, in these elements, since this power is given by the equation P=V2/R, where V is the supply voltage and R is the resistance of the heating element directly linked to the value of its resistivity.
These heating elements are, however, operated, to be fully on or fully off, i.e. to provide thermal safety that avoids all malfunction. Variation of the resistance is of the order of 25% between around 20° C. and 800° C., which permits generating power decreases of 25%, sufficient for standard tests.
Moreover, the heating wires currently used in heating elements for household electrical cooking appliances, in which the maximum temperature of the hotplates, or hotplates, is of the order of 300° C., present a variation of the order of 10% for wires of the Ni—Cr or Ni—Cr—Al type.
The PTC effect thus has only a small influence on the operation of the appliance. It appears, however, to take advantage of this effect, for purposes of protection and/or regulation of the appliances.
U.S. Pat. No. 2,767,288 discloses a heating element having a heating wire with a temperature coefficient at least equal to 0.003. The heating element offers an improvement in heat transfer at the level of heating element, through a double tube, the interior tube being made of a material having a high thermal conductivity, such as copper, and the outer tube being resistant to corrosion.
If such an element permits an automatic limitation of the power when the temperature rises, its use is limited to a substantial temperature range and for a heating of objects located at a distance from the heating element, or in contact with the material to be heated only at certain points.
Production of such a heating element remains however difficult due in part to the materials used for the two tubes. In addition, such an arrangement has the disadvantage of allowing only poor contact between the heating element and the material to be heated.